Invention relates to fabrication of films for highly effective field emitters of electron which can be used to produce flat panel displays, electron microscopes, microwave electronics, light source and for some other applications.
Cold emission film cathode is known which comprises a substrate coated with a diamond film [Application of Diamond Films and Related Materials: Third International Conference, 1995, NIST Special Publication 885, Edited by A. Feldman et al., p.37, p.61]. But because of a low density of emitting centers the film cathode on a basis of polycrystalline diamond films is not a highly effective emitter.
The most relevant to the present invention is a cold emission film cathode comprising a substrate coated with a carbon film [xe2x80x9cDiamond based field emission flat panel displaysxe2x80x9d Solid State Techn., 1995, May, p.71]. The film deposited on the substrate is a film of amorphous carbon
A method is known to produce a cold emission film cathode by a method of laser sputtering [xe2x80x9cDiamond based field emission flat panel displaysxe2x80x9d Solid State Techn. 1995, May, p,71], which comprises the deposition on a cold substrate of carbon evaporated from a graphite target by powerful laser radiation. Shortcoming inherent to this method is its complexity, high cost, limited scaling up capability, and also the low density of emitting centers (about 1000 per sq.cm in the electrical field of 20 V/micron) what is apparently insufficient for creation of a film color display with 256 grades of brightness.
A method is known to produce a cold emission film cathode by a method of plasma chemical vapor deposition comprising a DC glow discharge in an electrode gap between cathode and anode filled with hydrogen, beating a substrate up to the deposition temperature, injection of a carbon containing gas into flow and deposition of a film from a mixture of hydrogen and carbon containing gas, removing of excessive graphite phase by a discharge in hydrogen flow [A. T. Rakhimov, D. V. Selcznev, N. V. Suetin et al. Applications of Diamond Films and Related Materials: Third International Conf., Gaithersburg, Md., USA, 1995, NISTIR 5692, Supplement NIST Special Publication 885, p.11g]. Thus the nano-diamond film cathodes are produced. But the diamond films produced by this method grow very slowly and often do not possess emissive properties sufficient for creation of a full color display.
A method is known to produce a carbon cathode [USSR Authorship Certificate No 966782, IPC, publ.] comprising deposition of carbon filamentary crystals from a mixture of hydrogen, carbon dioxide and methane in the ratio of 15-25/74-83/1-2 and deposition of the carbon phase on a substrate at a temperature of 1200-1500xc2x0 C. But so produced films possess rather non uniform emissive performances and low density of emitting centers.
Requirements to the modern devices require the creation of a cold emission film cathode possessing high electron emissive performances and resistant to strong electrical fields, which can be used as a field emitter of electrons for production of flat panel displays, electron microscopes, microwave electronics, light sources and for some other applications.
This task can be solved due to that the cold emission film cathode is made as a substrate coated with a carbon film deposited on it in the form of a structure of irregularly located carbon micro- and nano-ridges and/or micro- and nano-threads normally oriented to the substrate surface and having typical size of 0.005-1 micron and density of 0.1-100 xcexcmxe2x88x922, and additionally coated with the second carbon film in the form of nano-diamond film which thickness is 0.1-0.5 microns.
Method of fabrication of the cold emission film cathode comprises a sequential deposition of two carbon films: carbon nanotip film on a substrate placed on an anode, due to a DC glow discharge in a mixture of hydrogen and carbon containing gas; and deposition of nano-diamond film above the grown graphite film. Deposition of nano-diamond film above the grown graphite film is made either in the same DC discharge or via a technology employing a hot filament as an activator of the process.
On a first stage a DC glow discharge is ignited at a current density of 0.15-0.5 A/sq.cm, and deposition is carried out in a mixture of hydrogen and carbon containing admixtures at a total pressure of 50-300 Torr, and in particular the ethyl alcohol vapor at concentration of 5-15% or methane at concentration of 6-30% and substrate temperature of 600-1100xc2x0 C. Also vapors of other carbon containing admixtures can be used, provided that a molar carbon content shall be retained. The gas mixture can be dissolved with an inert gas, e.g. argon, up to 75% at keeping the total pressure unchanged. Deposition of second carbon layer of the nanodiamond structure can be made by depositing of it from plasma of a DC discharge at the same parameters but reducing the concentration of carbon containing admixture down to 0.5-4% or deposition of nanodiamond layer can be made by chemical vapor deposition comprising a heating of metallic filament-activator up to temperature of 1800-2500xc2x0 C., and of a substratexe2x80x94up to temperature 600-1100xc2x0 C., and depositing a film in a mixture of hydrogen and carbon containing admixture at a concentration of 0.5-10% through a grid screen placed between the filament and substrate.
If concentration of ethyl alcohol vapor concentration is less than 5% or methane concentration is less than 6% and pressure is less than 50 Torr, then a nucleation is slowed down leading to a higher non uniformity of the emission characteristics. Also a texture of films changes. If concentration of ethyl alcohol vapor concentration is more than 15% or methane concentration is more than 30% and pressure is more than 300 Torr, then discharge is subjected to instability. If current density exceeds 0.5 A/sq.cm the gas and substrates are subjected to an overheating resulting in a worsening of a film emission performances.
If current density is less than 0.15 A/sq.cm the required activation of gas media is not provided.
Substrate temperature less than 600xc2x0 C. or more than 1100xc2x0 C. results in a pronounced change of the film texture and loss of emissive properties.
In case if a DC discharge is used on a second stage and if a carbon containing admixture concentration is more than 4% then a globule-like carbon film grows on the substrate surface with typical size of globules of more than 1 micron and having extremely poor emissive performances.
If a carbon containing admixture concentration is less than 0.5% then an abrupt decreasing of growth rate or even etching of the film grown on a first stage takes place.
In case if a hot filament activation is used on a second stage and if its temperature is less than 1800xc2x0 C. then no needed activation of the gas occurs. If its temperature is more than 2500xc2x0 C. then the filament life is too short. If substrate is heated up to a temperature less than 600xc2x0 C. or more than 1100xc2x0 C. then either graphite or film with unacceptably poor emissive performances is produced.